Move the general-vs-memory hook ownership logic out of the memory collector into
PluginLedgerMixin (_drop_fallback_hooks / _register_fallback_hook) so the collector
and the loader each call one manager method instead of reaching into manager privates.
Hoist hashlib to module scope. Trim the new suite to the three invariant cases
(run-once across load orders, distinct sources not suppressed, re-exported register).